Charging apparatus and image forming apparatus

ABSTRACT

There is provided a charging apparatus having high durability, in which no rusts etc. are generated, and by which a potential of a photoreceptor drum being charged can be controlled stably within an appropriate range for a long period of time owing to hardly impaired controllability of the potential of the photoreceptor drum being charged even with a some amount of contaminants such as toner that may be deposited on, and which is inexpensive as well. A charging apparatus includes a needle electrode, a holding member, two cleaner members, a support member, a moving member, a shield case, and a platy grid. On the surface of the needle electrode is formed a nickel layer containing boron.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a charging apparatus and an imageforming apparatus.

2. Description of the Related Art

In electrophotographic image forming apparatuses such as copyingmachines, printers and facsimile units, images are formed as follows. Byuse of a photoreceptor drum on a surface of which a photosensitive layercontaining a photoconductive substance is formed as an image carrier,the surface of the photoreceptor drum is subjected to application ofcharges so as to be uniformly charged therewith. Subsequently,electrostatic latent images corresponding to image information areformed by various image forming processes. The electrostatic latentimages are developed by a toner-containing developer supplied from adeveloping unit, so as to obtain visible images which are thentransferred onto a recording material such as paper. After that, thevisible images are fixed on the recording material by heat and pressuregiven by a fixing roller so that an image is formed on a recordingpaper.

In the image forming apparatus as described above, a charging apparatusis used for charging the surface of the photoreceptor drum. The chargingapparatus is composed of: an electrode for conducting corona dischargeon the photoreceptor drum; a grid which is optionally disposed betweenthe surface of the photoreceptor drum and the electrode, for controllingan amount of charges imparted from the electrode to the surface of thephotoreceptor drum, and thus controlling a potential of the surface ofthe photoreceptor drum being charged; a support member for supportingthe electrode and the grid. Since the grid is capable of almost exactlycontrolling the potential of the surface of the photoreceptor drum beingcharged, a charging apparatus provided with the grid is becomingpredominant at present. As the grid, for example, there are used a wiregrid which is prepared from stainless steel or tungsten, and a porousplate grid which is formed of a metal plate (grid substrate) ofstainless steel or the like with a large number of perforations.

As the electrode for the charging apparatus, for example, there are useda wire electrode and a metal plate electrode (hereinafter referred to as“needle electrode”) having a plurality of needle portions. Among them,the needle electrode is preferably used because of its advantages suchas a less number of components, a longer operating life, less generationamount of ozone, and less frequent failures by virtue of nodisconnection. The needle electrode is manufactured by etching a metalplate which is mainly formed of an iron-based metal material such asstainless steel, to form a plurality of needle portions in the metalplate. The needle electrode manufactured by etching is also referred toas an etched electrode. An etched cross section of the needle electrodelacks in the smoothness. Further, a plurality of edges for conductingdischarge are present at the top end of the needle portion, and theshape of the edges present at the top ends of a plurality of needleportions is not uniform. Accordingly, discharges from the respectiveneedle portions are not uniform. As a result, it is not possible tosufficiently control the potential of the surface of the photoreceptordrum being charged, so that the potential of the charged surface of thephotoreceptor drum becomes uneven.

Further, the material for the needle electrode, i.e. the iron-basedmetal material such as stainless steel has high durability, but involvesa drawback of being easily oxidized due to water content under a highhumidity circumstance and ozone generated by corona discharging during acharging operation. In addition, in a long-time use of the needleelectrode, a use of the needle electrode under the high humiditycircumstance, a contact of the needle electrode with ozone, etc. areinevitable. Accordingly, the needle electrode formed of a metal materialsuch as stainless steel corrodes due to moisture in air or due to ozoneetc., resulting in deterioration of durability thereof. In addition,there is lowered controlling performance for high voltage applied to theneedle electrode for conducting the corona discharge from the needleportion, with the result that the potential of the discharged surface ofthe photoreceptor drum is uneven, leading a to-be-solved problem thatthe surface of the photoreceptor drum cannot be constantly charged to adesired potential in a stable manner.

Further, as in the case of the needle electrode, the wire electrode alsoinvolves a to-be-solved problem such that rust or corrosion occurs dueto ozone generated by corona discharge, with the result that thepotential of the charged surface of the photoreceptor drum is uneven.

In view of the foregoing problems in the charging apparatus, there hasbeen proposed, for example, a charging apparatus including: a wireelectrode extended in a shield case having one surface thereof open; anda plate grid arranged between the wire electrode and the photoreceptordrum, the plate grid which is formed by applying a nickel plated layerof about 1 μm thickness on a surface of a porous stainless steel plateand further forming thereon a gold plated layer of about 0.3 μmthickness (refer to, for example, Japanese Unexamined Patent PublicationJP-A 11-40316 (1999)). In the plate grid of JP-A 11-40316, the goldplated layer is formed by way of the nickel plated layer, with theresult that the gold plated layer is less easily peeled off, and thecorrosion resistance and the controllability of the potential of thesurface of the photoreceptor drum being charged are relativelysatisfactory. However, the manufacture of the plate grid requires twoplating steps, i.e., nickel plating and gold plating. This imposesdrawbacks such as more complicated manufacturing step and increase ofthe cost. Further, in order to make this plate grid sufficiently exhibitthe preferred characteristics as described above, a thickness of thegold metal layer needs to be set at 0.3 μm or more. In addition, sincethe plate grid is a relatively large member having a substantially thesame size as the photoreceptor drum, the usage of gold is necessarilyincreased also because of the necessity for increasing the thickness ofthe plated layer. However, such a heavy usage of gold unnecessarilyincreases the cost of the charging apparatus and thus the cost of theimage forming apparatus, resulting in loss of the general applicabilityof the image forming apparatus based on its relatively low price, whichis one of the advantages of the image forming apparatus. Accordingly,there has been a demand for realizing, without using an expensivematerial such as gold, a charging apparatus having the needle electrodeand the plate grid which are excellent in durability and controllabilityof the potential of the surface of the photoreceptor drum being charged.

Further, there has been proposed a charging apparatus including: a wireelectrode; and a plate grid which is obtained by forming a gold platedlayer directly on a surface of a stainless steel metal plate by anelectrolytic plating method using a pulse current, as in the case ofJP-A 11-40316 (refer to, for example, Japanese Unexamined PatentPublication JP-A 2001-166569). Also in this plate grid electrode, thegold plated layer is less easily peeled off and, like the plate grid inJP-A 11-40316, the corrosion resistance is high and the controllabilityof the potential of the surface of the photoreceptor drum being chargedare also favorable. However, also in this plate grid, a thickness of thegold metal layer needs to be set at 0.3 μm or more, it involves the samedrawbacks as those in the charging apparatus of JP-A 11-40316.

Further, there has been a corona charging apparatus having a wireelectrode, at least a shaft of which is formed of iron-boron-basedamorphous metal (refer to, for example, Japanese Unexamined PatentPublication JP-A 61-98368 (1986)). There is an advantage that a use ofthe wire electrode of JP-A 61-98368 can decrease unevenness in charge onthe surface of the photoreceptor drum. However, even such a wireelectrode does not overcome the drawback of being easily oxidized due toozone, to a sufficiently satisfactory level. Accordingly, in order toimpart long-lasting durability to the wire electrode, a coating layerformed of metal or the like material must be formed on a surface of theshaft formed of iron-boron-based amorphous metal.

Further, in all of the charging apparatuses of JP-A 11-40136, JP-A2001-166569, and JP-A 61-98368, in the case of forming images by use oftoner which contains as an external additive silica having a surfacethereof hydrophobized by trimethylsilyl group-containing-polysiloxanes(hereinafter referred to as “hydrophobic silica” unless particularlymentioned), the polysiloxanes are attached to the plate grid or the wireelectrode. Due to the foregoing, the above charging apparatuses thushave drawbacks of easily causing charging defects. At present, the tonercontaining the hydrophobic silica is an essential constituent forspeeding up image formations in an electrophotographic image formingapparatus.

On the other hand, it has also been proposed to coat the electrode withgold (refer to, for example, Japanese Unexamined Patent Publication JP-A2004-4334). The charging apparatus of JP-A 2004-4334 includes a needleelectrode on a surface of which a coating layer made of gold, platinum,copper, nickel or chromium is formed by plating. In JP-A 61-98368, amethod of etching, precision pressing, or the like method is used forforming the needle electrode, but the cross section of the needleelectrode obtained by the method lacks in smoothness and results in fineirregularities. Accordingly, even after applying the plating, fineirregularities on the cross section remain as they are and may disturbthe balance of the corona discharge, resulting in uneven potential ofthe charged surface of the photoreceptor drum. Further, contaminantssuch as toner are easily deposited on the fine irregularities. That is,the needle electrode of JP-A 2004-4334 has a drawback that contaminantssuch as the toner are attached thereto during long time use and thiscauses the charged surface of the photoreceptor to have further unevenpotential.

SUMMARY OF THE INVENTION

An object of the invention is to provide a charging apparatus havinghigh durability, by which a potential of a photoreceptor drum beingcharged can be controlled stably within an appropriate range for a longperiod of time owing to hardly impaired controllability of the potentialof the photoreceptor drum being charged even with a some amount ofcontaminants such as toner, particularly toner containing hydrophobicsilica, that may be deposited on, and in which the depositedcontaminants can be easily removed, and which is inexpensive, as well asan image forming apparatus including the charging apparatus, which iscapable of recording images of high quality for a long period of time.

The invention provides a charging apparatus comprising:

an electrode having a plurality of pointed protrusions, that applies avoltage to a surface of a photoreceptor drum to charge the surface; and

a platy grid disposed between the electrode and the photoreceptor drum,that controls a potential of the surface of the photoreceptor drum beingcharged,

wherein a nickel layer containing boron is formed on at least one ofsurfaces of the electrode.

According to the invention, there is provided a charging apparatuscomprising a platy needle-shaped electrode (hereinafter referred to as“a needle electrode”) and a platy grid. The needle electrode has aplurality of pointed protrusions and applies a voltage to a surface of aphotoreceptor drum. On a surface of the needle electrode is formed anickel layer containing boron (hereinafter referred to as “aboron-containing nickel layer” unless particularly mentioned). The platygrid is disposed between the needle electrode and the photoreceptordrum, and controls a potential of the surface of the photoreceptor drumbeing charged. Owing to structural features such that the needleelectrode has a plurality of pointed protrusions and a surface of theneedle electrode is coated with the boron-containing nickel layer,corona dischargeability toward the surface of the photoreceptor drum isexcellent, and the potential of the surface of the photoreceptor drumbeing charged can be controlled so as to fall within an appropriaterange and so as to be uniform. Moreover, the needle electrode has highdurability, and is capable of exerting the above-described chargingcontrollability of potential in a stable manner over a long period oftime. Furthermore, the needle electrode has a surface thereof coatedwith the boron-containing nickel layer whereby contaminants such astoner deposited on the surface can be easily removed by a commonly-usedcleaning device. Accordingly, an image forming apparatus provided with acharging apparatus having the needle electrode is capable of recordingimages of high quality for a long period of time. Particularly in thecase of forming images by use of toner containing hydrophobic silica asan external additive, the image forming apparatus exhibits excellentdurability. Furthermore, the needle electrode is constructed by formingon a surface of a substrate not a gold plated layer which is used in therelated art but a boron-containing nickel plated layer only, resultingin an advantage of lower cost compared to the needle electrode of therelated art. The charging apparatus of the invention can be preferablyused, owing to its excellent durability against ozone, particularly in atandem type color image forming apparatus which generates a large amountof ozone when a plurality of charging apparatuses are driven at the sametime.

Further, in the invention, it is preferable that the nickel layercontaining boron is formed by an electroless plating method.

According to the invention, the boron-containing nickel layer is formedon the surface of the needle electrode by the electroless platingmethod. Compared to the boron-containing nickel layer obtained by anormal electrolytic plating method using DC electricity, theboron-containing nickel layer obtained by the electroless plating methodhas a dense and hard structure with less pinholes, of which layerthickness is thin and nevertheless uniform, and has high adhesion withthe needle electrode substrate. Accordingly, the boron-containing nickellayer makes the rough surface of the needle electrode smooth so that thecharging controllability of potential and durability of the needleelectrode plate are further enhanced. Furthermore, the contaminants suchas toner are less easily deposited on the needle electrode plate.

Further, in the invention, it is preferable that a thickness of thenickel layer containing boron is 0.3 μm or more.

According to the invention, a thickness of the boron-containing nickellayer in the needle electrode plate is set at 0.3 μm or more, with theresult that the charging controllability of potential and durability ofthe needle electrode plate are securely exhibited.

Further, in the invention, it is preferable that another nickel layer isformed between the electrode and the nickel layer containing boron.

According to the invention, in the needle electrode, another nickelplated layer is formed between the electrode and the nickel layercontaining boron, with the result that the boron-containing nickel layeris further prevented from being peeled off from the needle electrodesubstrate, resulting in enhancement in durability of the needleelectrode. Accordingly, it is possible to obtain the charging apparatuswhich is capable of stably controlling the potential of the surface ofthe photoreceptor drum being charged over a longer period of time.

Further, in the invention, it is preferable that the nickel layercontaining boron contains phosphorus together with boron.

According to the invention, in the needle electrode, the nickel layercontaining boron contains phosphorus together with boron, with theresult that the layer has enhanced adhesion with the needle electrodesubstrate and thus the needle electrode has further enhanced durability.

The invention provides an image forming apparatus comprising:

a photoreceptor drum on a surface of which an electrostatic latent imageis formed;

one of the charging apparatuses described above, for charging thesurface of the photoreceptor drum;

an exposure unit that irradiates the charged surface of thephotoreceptor drum with signal light based on image information tothereby form the electrostatic latent image;

a developing apparatus that develops the electrostatic latent imageformed on the surface of the photoreceptor drum to thereby form a tonerimage;

a transfer unit that transfers the toner image onto a recordingmaterial; and

a fixing unit that fixes the toner image transferred on the recordingmaterial.

According to the invention, in the image forming apparatus composed ofthe photoreceptor drum, the charging apparatus, the exposure unit, thedeveloping apparatus, the transfer unit, and the fixing unit, the use ofthe charging apparatus of the invention having the needle electrode on asurface of which the boron-containing nickel layer is formed, makes itpossible to stably keep within an appropriate range the potential of thesurface of the photoreceptor drum being charged during formation of theelectrostatic latent images. Accordingly, images of high quality can berecorded over a long period of time and moreover, there is no goldplated layer which is contained in the related art, with the result thatan inexpensive image forming apparatus can be obtained.

Further, in the invention, it is preferable that the image formingapparatus further comprises a cleaning unit that cleans the surface ofthe photoreceptor drum after the toner image has been transferred ontothe recording material by the transfer unit,

wherein the developing apparatus and/or the cleaning unit are/is locatedabove the charging apparatus.

According to the invention, there is provided an image forming apparatusfurther comprising the cleaning unit for cleaning the surface of thephotoreceptor drum, in which the developing apparatus and/or thecleaning unit are/is located above the charging apparatus. Particularlyin the case of designing the tandem type color image forming apparatus,the constitution in which the developing apparatus and the cleaning unitare disposed above the charging apparatus, can contribute to moresimplified constitution and reduced size of the image forming apparatus.This constitution has a drawback such that a charging defect is easilycaused on the surface of the photoreceptor drum. However, in the imageforming apparatus using the charging apparatus of the invention, evenwith the constitution that the developing apparatus and/or the cleaningunit are/is located above the charging apparatus, the generation ofcharging defect is remarkably suppressed so that images of high qualityand high grade can be formed over a long period of time.

Further, in the invention, it is preferable that the toner image isformed of toner which contains hydrophobic silica as an externaladditive.

According to the invention, in the image forming apparatus of theinvention, even when the toner containing hydrophobic silica as anexternal additive is used in order to speed up image formation, thecharging defect attributable to deterioration of the charging apparatusis hardly caused, so that images of high quality and high grade can beformed over a long period of time.

BRIEF DESCRIPTION OF THE DRAWINGS

Other and further objects, features, and advantages of the inventionwill be more explicit from the following detailed description taken withreference to the drawings wherein:

FIG. 1 is a perspective view schematically showing a configuration of acharging apparatus according to one embodiment of the invention;

FIG. 2 is a front view of the charging apparatus shown in FIG. 1; and

FIG. 3 is a sectional view schematically showing a configuration of animage forming apparatus according to another embodiment of theinvention.

DETAILED DESCRIPTION

Now referring to the drawings, preferred embodiments of the inventionare described below.

FIG. 1 is a perspective view schematically showing a configuration of acharging apparatus 1 according to one embodiment of the invention. FIG.2 is a front view of the charging apparatus 1 shown in FIG. 1. Thecharging apparatus 1 includes a needle electrode 2, a holding member 3,two cleaner members 4 a and 4 b, a support member 5, a moving member 6,a shield case 7, and a platy grid 8. The needle electrode 2 is a platymember having a plurality of pointed protrusions 10. The holding member3 holds the needle electrode 2. The cleaner members 4 a and 4 b aredisposed so as to be movable relatively to the needle electrode 2, andclean a surface of the needle electrode 2 by frictionally rubbing theneedle electrode 2 upon movement. The support member 5 supports thecleaner members 4 a and 4 b. The moving member 6 moves the cleanermembers 4 a and 4 b and the support member 5. The shield case 7 containsthe needle electrode 2, the holding member 3, the cleaner members 4 aand 4 b, and the support member 5. The platy grid 8 adjusts a potentialof a surface of photoreceptor drum (not shown) being charged. Thecharging apparatus 1 is disposed along a longitudinal direction of thephotoreceptor drum (not shown) so as to face the photoreceptor drum.

The needle electrode 2 is a thin plate member, for example, made ofstainless steel, which is constituted by forming a boron-containingnickel layer over a surface of a needle electrode substrate composed ofa flat plate 9 extending longitudinally in one direction and the pointedprotrusions 10 formed so as to transversely protrude from one end facein a transverse direction of the flat plate 9. Referring, for example,to a size of the needle electrode 2, a length L1 in the transverselength of the flat plate 9 is preferably about 10 mm while a length L2in the protruding direction of the protrusion 10 is preferably about 2mm, a radius of curvature R at a top end of the protrusion 10 ispreferably about 40 μm, and a pitch TP at which the protrusions 10 areformed is preferably about 2 mm.

The needle electrode 2 can be manufactured in accordance with aheretofore known method. One example of the method is a manufacturingmethod including a chemical polishing step, a water washing step, anacid dipping step, a water washing step, a pure water dipping step, anickel layer forming step, a boron-containing nickel layer forming step,a water washing process, and a drying process. Among the steps, thenickel layer forming step is not an essential step, but optionallycarried out. The nickel layer is formed, for example, by a generalplating method.

In the chemical polishing step, masking and etching are performed sothat a plurality of pointed protrusions are formed in a plate metal. Themasking can be conducted in accordance with a heretofore known method.The etching can also be conducted in accordance with a heretofore knownmethod, which includes, for example, a method of spraying an etchingsolution such as an aqueous solution of ferric chloride to the platemetal. As the metal for the material of the plate metal, any materialscan be used without a particular restriction unless the material canconduct the corona discharge when a voltage is applied thereto, and canbe subjected to formation of a pointed protrusion and plating. Examplesof the material include, for example, stainless steel, aluminum, nickel,copper and iron. Among them, stainless steel is preferred. Specificexamples of the stainless steel include, for example, SUS304, SUS309 andSUS316. Among them, SUS304 is preferred. A thickness of the plate metalis, without particular restriction, preferably from 0.05 to 1 mm andmore preferably from 0.05 to 0.3 mm.

The plate metal in which the plurality of pointed protrusions are formedin the chemical polishing step, is treated with water washing, acidcleaning or pure water cleaning in the water washing step, the aciddipping step, the water washing step and the pure water dipping step,whereby obstacles are removed from the surface so that a needleelectrode substrate is obtained.

In a boron-containing nickel layer forming step, the boron-containingnickel layer can be formed in accordance with an electroless nickelplating method including a catalytic nickel plating method (Kanigenprocess) as disclosed in Japanese Examined Patent Publication JP-B23027515.

As a plating solution applied to the boron-containing nickel layer,there is used a plating solution prepared by adding as a reducing agentboron-containing compounds such as dimethylamine borane [(CH₃)₂NHBH₃],sodium borohydride (NaBH₄), dimethylamine boron, and diethylamine boronto an aqueous solution containing nickel salts (such as nickel sulfate,nickel chloride, nickel acetate and nickel carbonate). The platingsolution is adjusted so that pH thereof is preferably in a range ofabout 6 to 7 and more preferably around 6.2. An amount of the boroncompounds added to the plating solution is also not particularlyrestricted, and the boron-containing nickel layer being formed may beadjusted so as to preferably contain 0.1 to 1.0% by weight and morepreferably 0.2 to 0.5% by weight of boron. The plating solution maycontain an appropriate amount of phosphoric acid and salt thereof.Examples of phosphoric acid and salt thereof include hypophosphorousacids and salts thereof such as hypophosphorous acid, sodiumhypophosphite, potassium hydpophosphite, and nickel hypophosphite. Acommercially available plating solution for boron-containing nickel canalso be used. A liquid temperature of the plating solution forboron-containing nickel is preferably 30 to 95° C. or higher and more,preferably 80 to 95° C.

Into the plating solution for boron-containing nickel layer of whichcomposition, pH, and liquid temperature are as described above, theneedle electrode substrate is dipped to be subjected to the electrolessplating. The boron-containing nickel plated layer is thus formed on thesurface of the substrate.

A content of boron in the boron-containing nickel plated layer is, asdescribed above, preferably 0.1 to 1.0% by weight and more preferably0.2 to 0.5% by weight to an entire amount of the plated layer, and aremaining part thereof is formed of nickel. In the case where theboron-containing nickel plated layer contains phosphorus together withboron, a content of boron is the same as the content stated above whilea content of phosphorus is preferably 0.5 to 3% by weight and morepreferably 1.0 to 2.0% by weight to an entire amount of the platedlayer, and a remaining part thereof is formed of nickel.

A layer thickness of the boron-containing nickel plated layer is,without a particularly restriction, preferably 0.3 μm or more and morepreferably 0.3 to 20 μm and particularly preferably 4 to 20 μm. Sincethe liquid agent of the plating solution is deposited uniformly on thesurface of the needle electrode substrate, the boron-containing nickelplated layer formed by electroless plating has a favorablecharacteristic that the layer is uniform with no variation in thicknessand with the extremely small number of pinholes even in the case wherethe plating layer is as thin as about 0.3 μm. Further, the platingstructure is dense and has high adhesion with the surface of the needleelectrode substrate and even when used for a long time, no peeling orthe like is caused.

In the case where the thickness is less than 0.3 μm, there are easilyformed the pinholes which make the structure uneven and through whichstainless steel of the needle electrode substrate is corroded. Thisleads a deteriorated releasing property, which causes deposition ofattachment on tips of the needle electrode (sawtooth electrode), and thepotential for charging is thus liable to be partially unstable. On theother hand, if the thickness is much larger the 20 μm, plated membranesmay possibly be peeled off by stress. Since the thickness of theboron-containing nickel plated layer is substantially in proportion withthe plating time, a plated layer of a desired thickness may be obtainedby appropriately changing the dipping time of the needle electrodesubstrate into the plating solution.

The needle electrode (sawtooth electrode) produced by photoetching whichis employed to give the tips sharpness, has an etched surface thereofrough, when observed by electron microscopy, with minute irregularitiesformed by the crystal grain boundary. The plated layer having athickness of 4 to 20 μm has a more effect of absorbing such minuteirregularities to make the surface smoother. However, even when thethickness of the plated layer is secured, there is formed an irregularor granular surface just like a wall surface of limestone cave. Theirregular or granular surface will be reduced when the liquidtemperature of plating solution is controlled so as to be above 80° C.In the case where the surface is irregular or granular, if a chargingdefect is caused by an extraneous substance attached to the tip of theneedle electrode (sawtooth electrode) in an unexpected situation, theextraneous substance may not be fully removed even by use of a cleanerdisposed in the charging apparatus, the cleaner which is a scrap sheetmade of synthetic resin (such as polyethylene terephthalate) for rubbingand cleaning a needle electrode surface. Such a charging defect maypossibly lead an unrecoverable state. When an appropriate platingcondition is selected to form a plated layer having a thickness of 10μm, for example, the plated layer has a very smooth surface, on a tip ofwhich an attached extraneous substance can be easily removed by thecleaner so that a charging performance can be recovered.

The boron-containing nickel layer forming step can be conducted byelectric plating. As the plating bath, it is possible to use a platingbath of the same sort as that used for the electroless plating.Conditions for the electric plating are the same as those for acommonly-used electric nickel plating. The boron-containing nickelplating through electric plating has a trend inherent to electricplating, that is, plating tends to be applied to the edge portions. Itis therefore necessary to increase the layer thickness in order to makethe thickness of the plating layer uniform, and the layer thicknessneeds to be preferably 3 μm or more.

In the case of forming the boron-containing nickel layer, electrolessplating or electric plating is selected in accordance with the featureand cost of the respective plating methods.

The holding member 3 holding the needle electrode 2 is a member whichextends longitudinally in one direction as the needle electrode 2 is,and of which cross section in perpendicular to the longitudinaldirection has an inverted T shape. The holding member 3 is made ofresin, for example. The needle electrode 2 is screwed by thread members11 at near both ends in the longitudinal direction of the needleelectrode 2 onto one lateral side of a protruded portion of the holdingmember 3. For charging the photoreceptor drum 19, a voltage at about 5kV is applied for generating corona discharge during operation.

Each of the cleaning members 4 a and 4 b has a plate-like shape, morespecifically, a T-shaped configuration when projected on a plane, and ismade of an elastic body of a metal material or a polymeric material witha thickness t of from 20 to 40 μm. In the case where the thickness t isless than 20 μm, the member is easily deformed upon abutting against theneedle electrode 2, but a reaction force accompanied by the deformation,i.e., the pressing force to the needle electrode 2, is small, with theresult that contaminants deposited on the needle electrode 2 cannot beremoved sufficiently. In the case where the thickness t exceeds 40 μm,the contaminants deposited on the needle electrode 2 can be removedsufficiently, but higher stiffness excessively increases the pressingforce to the needle electrode 2, with the result that the tip of theprotrusion 10 of the needle electrode 2 may possibly be fractured by thedeformation. As a result, in the case where the thickness t is out ofthe range of 20 to 40 μm, image unevenness etc. due to the chargingdefect may possibly be caused. As the metal material constituting thecleaner members 4 a and 4 b, phosphor bronze, ordinary steel, stainlesssteel, etc. can be used. Among them, stainless steel is preferred from aviewpoint of the duration life based on anti-oxidation property whileconsidering that the cleaner members 4 a and 4 b are used in theatmosphere of ozone generated by corona discharge. Typical examples ofthe stainless steel include, for example, austenitic stainless steel,i.e. SUS304 and ferritic stainless steel, i.e. SUS430, which are definedby Japanese Industrial Standard (JIS) G4305. It is however possible touse other kinds of stainless steel without restricting to the abovematerials.

The hardness of the cleaner members 4 a and 4 b is preferably 115 ormore by Rockwell hardness M scale according to American Society forTesting and Materials (ASTM) Standards D785. In the case where theRockwell hardness is less than 115, the material is excessively soft andtherefore, the cleaner members 4 a and 4 b are deformed excessively thanrequired when abutting on and frictionally rubbing the needle electrode2, failing to obtain the cleaning effect. Since no particular problemoccurs in view of the function in the case where the hardness of thecleaner members 4 a and 4 b is high, it is not necessary to define theupper limit. However, since the upper limit value in the Rockwellhardness M scale is 130, the upper limit, if defined, is 130.

The lateral size w of the longitudinal rod portion of the T-shape of thecleaner members 4 a and 4 b, which is a portion abutting against theneedle electrode 2, that is to say, the size w of the cleaner members 4a and 4 b in the direction vertical to the moving direction of thecleaner members 4 a and 4 b and in the direction vertical to theextending direction of the protrusion 10, is preferably 3.5 mm or more.In the case where the lateral size w is less than 3.5 mm, the value perunit area of a force generated upon deformation when pressed by theneedle electrode 2 is large, and it therefore becomes easier to causefatigue fracture due to the repetitive deformation, resulting in adecrease in length of the duration life. The value per unit area of theforce described above can be decreased to extend the duration lifeagainst the repetitive deformation by making the lateral size w to 3.5mm or more. However, the excessively increased width makes the stiffnesstoo high and the size of the device too large, and it is thereforepreferable to set the upper limit to about 10 mm.

The cleaner members 4 a and 4 b and the needle electrode 2 arepreferably arranged such that an intrusion amount of the protrusion 10of the needle electrode 2 to the cleaner members 4 a and 4 b is from 0.2to 0.8 mm. The intrusion amount d means an overlap length between thecleaner members 4 a and 4 b and the protrusion 10 in the extendingdirection of the protrusion 10 in a state where the cleaner members 4 aand 4 b and the protrusion 10 are projected upon a virtual planeperpendicular to a moving direction of the cleaning members 4 a and 4 brelatively to the needle electrode 2. In the case where the intrusionamount d is less than 0.2 mm, a reaction force accompanied by thedeformation, i.e., the pressing force to the needle electrode 2, issmall, with the result that containments deposited on the needleelectrode 2 cannot be removed sufficiently. In the case where theintrusion amount d exceeds 0.8 mm, the contaminants deposited on theneedle electrode 2 can be removed sufficiently, but the reaction forceaccompanied by the deformation (i.e. the pressing force to the needleelectrode 2) is too large, with the result that the tip of theprotrusion 10 of the needle electrode 2 may possibly be fractured by thedeformation. As a result, in the case where the intrusion amount d isout of the range from 0.2 to 0.8 mm, image unevenness etc. due to thecharging defect may possibly be caused.

The support member 5 is a member having an inverted L-shapedconfiguration for supporting the cleaner members 4 a and 4 b. To a beamportion of the support member 5 is attached arm portions of the cleanermembers 4 a and 4 b in the T-shaped configuration. The two cleanermembers 4 a and 4 b are disposed so as to have a predetermined gap L3with respect to the direction moving relatively to the needle electrode2. The gap L3 is selected so as to have a distance such that when onecleaner member 4 a is deformed upon abutting against the needleelectrode 2, the other cleaner member 4 b is not in contact with thedeformed cleaner member 4 a, and this is adjustable by the thickness ofthe beam portion of the support member 5 to which the cleaner members 4a and 4 b are attached. Since a deformation state of the cleaner members4 a and 4 b changes depending on the material thereof, the gap L3 ispreferably determined by a previous testing of the deformation state ofthe material. In the case where each of the cleaner members 4 a and 4 bis made, for example, of stainless steel at a thickness t=30 μm, the gapL3 is preferably 2 mm. While one cleaner member 4 a frictionally rubsthe needle electrode 2, the gap L3 provided between the two cleanermembers 4 a and 4 b helps to enable the pressing force to be maintainedin a preferred range without hindrance in the deformation due to theother cleaner member 4 b. As a result, the needle electrode 2 can becleaned sufficiently with no deformation injury caused on the top endthereof.

The shield case 7 is made of, for example, stainless steel. The shieldcase 7 is a container-like member of which outer shape is a rectangularparallelepiped with an inner space and which has an opening in onesurface facing a photoreceptor drum (not shown). Further, the shieldcase 7 extends longitudinally in the same direction as the extendingdirection of the needle electrode 2. A cross sectional configuration ofthe shield case 7 in a direction perpendicular to a longitudinaldirection thereof is substantially U-shaped. Further, a holding member 3is attached to a bottom 15 of the shield case 7. Moreover, an end of acolumnar portion of the support member 5 is inserted slidably into agroove 14 which is formed by an inner lateral surface 13 of the shieldcase 7 and the holding member 3.

The columnar portion of the support member 5 has a through hole 12therein, which extends in parallel with the extending direction of theneedle electrode 2 and through which the moving member 6 is inserted.Since the moving member 6 is fixed to the support member 5 at a portionwhere the moving member 6 is inserted through the through hole 12,traction of the moving member 6 in the extending direction of the needleelectrode 2 makes the support member 5 move slidably with respect to thegroove 14 so that the support member 5 is guided by the groove portion14 to be thereby allowed to move in the extending direction of theneedle electrode 2. That is to say, the cleaner members 4 a and 4 bsupported by the support member 5 can be made to abut on andfrictionally rub the needle electrode 2.

The moving member 6 is a thread-like or wire-like member. The movingmember 6 extends from a hole or gap formed in the shield case 7 tooutside of the shield case 7 so that an end of the moving member 6 issuspended by way of an outer surface of the shield case 7 or by way ofpulleys 16 a and 16 b disposed on a machine body of copying machine. Thepulleys 16 a and 16 b and the end of the moving member 6 are not shownin FIG. 1. The end of the moving member 6 preferably extends as far asthe outside of the machine body of the image forming apparatus. Thisenables to clean the needle electrode 2 without detaching the chargingapparatus 1 from the image forming apparatus or without opening theimage forming apparatus.

When cleaning is conducted by making the cleaner members 4 a and 4 babut on the needle electrode 2 by means of traction of the moving member6, the pressing force of the cleaner members 4 a and 4 b against theneedle electrode 2 is adjusted preferably to 10 to 30 gf. In the casewhere the pressing force is less than 10 gf, contaminants such as toneror paper dust deposited on the needle electrode 2 can not possibly beremoved sufficiently and, on the other hand, in the case where thepressing force exceeds 30 gf, the top end of the protrusion 10 of theneedle electrode 2 may possibly be fractured by deformation.

The pressing force of the cleaner members 4 a and 4 b against the needleelectrode 2 can be adjusted, for example, as described below. The forceloaded on the cleaner member 4 a or 4 b is measured in a state where aweight is suspended from one end of the moving member 6. Measurement isconducted, for example, by connecting a spring balance to the cleanermember 4 a or 4 b. Then, by selecting a weight to provide a force of 10to 30 gf loaded on the cleaner member 4 a or 4 b and suspending thepre-selected weight to the end of the moving member 6 upon cleaning theneedle electrode 2, cleaning can be conducted under a predeterminedpressing force. Further, an electric motor of which a rotational torquehas been adjusted may be connected to the end of the moving member 6 sothat a predetermined pressing force can be loaded.

The platy grid 8 is located between the needle electrode 2 and thephotoreceptor drum (not shown). By applying the voltage to the platygrid 8, fluctuation in the charged state on the surface of thephotoreceptor drum (not shown) is adjusted to make the chargingpotential uniform. The platy grid 8 contains a metal material as in thecase of the needle electrode 2. Further, the platy grid 8 can bemanufactured in the same manner as that for the needle electrode 2except that masking and etching are performed in the chemical polishingstep so that the platy grid 8 is formed into a porous structure.Further, the same nickel plating, nickel boron-containing nickelplating, etc. can be applied to the platy grid 8 in the same manner asin the case of the needle electrode 2.

According to the charging apparatus 1, the application of the voltage tothe needle electrode 2 causes corona discharge whereby the surface ofthe photoreceptor drum (not shown) is charged, and the application of apredetermined grid voltage to the grid electrode makes the charged stateon the surface of the photoreceptor drum uniform, with the result thatthe surface of the photoreceptor drum (not shown) can be charged up to apredetermined potential and polarity. Further, by the traction of themoving member 6, the support member 5 and, correspondingly, the cleanermembers 4 a and 4 b which abut on the needle electrode 2, move so thatcontaminants such as toner deposited on the needle electrode 2 areremoved efficiently and reliably.

FIG. 3 is a sectional view schematically showing a configuration of animage forming apparatus 61 according to another embodiment of theinvention.

An image forming apparatus 61 is a multifunctional machine having acopying function, a printer function and a facsimile function together,and according to image information being conveyed to the image formingapparatus 61, a full-color or monochrome image is formed on a recordingmedium such as a recording paper. That is, the image forming apparatus61 has three types of printing modes, i.e., a copier mode (copyingmode), a printer mode and a FAX mode, and the printing mode is selectedby a control portion (not shown) depending on, for example, theoperation input from an operation portion (not shown) and reception ofthe printing job from an external host apparatus such as a personalcomputer.

The image forming apparatus 61 comprises a photoreceptor drum 19, acharging apparatus 1, an exposure unit 63, a developing apparatus 20, acleaning unit 64, a transfer unit 65, a fixing unit 66, a sheetconveying channel S, an automatic paper feed tray 67, a manual paperfeed tray 68, and a paper discharge station 69. That is to say, theimage forming apparatus 61 has a feature of including the chargingapparatus 1 of the invention.

The image forming apparatus 61 is a full-color printer, and thus adaptedfor image information in accordance with a cooler image formed by usingcolors of black (b), cyan (c), magenta (m), and yellow (y). Accordingly,for the respective colors, there are provided four sets of thephotoreceptor drums 19, the charging apparatuses 1, the developingapparatuses 20, the cleaning units 64, and the transfer rollers 71provided in the transfer units 65. The four sets of respective unitsprovided for the respective colors are distinguished herein by givingalphabets indicating the respective colors to the end of the referencenumeral, and in the case where the sets are collectively referred to,only the reference numeral is shown. Note that the exposure unit 63divaricates the image information therein into light of respective colorinformation for b, c, m, and y, and emits the light of respective colorinformation to thereby expose the photoreceptor drum 19 on which anelectrostatic latent image of respective colors is to be formed. As theexposure unit 63, there is thus provided one laser scanning unit having,for example, a laser emitting portion and a plurality of reflectingmirrors.

The charging apparatus 1, the developing apparatus 20, the transferroller 71, and the cleaning unit 64 are disposed in this order aroundthe photoreceptor drum 19. The charging apparatus 1 is disposed belowthe developing apparatus 20 and the cleaning unit 64 in a verticaldirection thereof. That is to say, the developing apparatus 20 and thecleaning unit 64 are disposed above the charging apparatus 1 in avertical direction thereof. The exposure unit 63 is disposed so that thelight of respective color information emitted from the exposure unit 63passes between the charging apparatus 1 and the developing apparatus 20to irradiate a surface of the photoreceptor drum 19 therewith.

The photoreceptor 19 is rotatably supported around an axis thereof by adriving mechanism (not shown), and includes a conductive substrate (notshown) in cylindrical, circular columnar shape or in the form of thinfilm sheet, preferably, a cylindrical conductive substrate, and aphotosensitive layer formed on the surface of the conductive substrate.

As the conductive material for the material of the conductive substrate,those customarily used in the relevant field can be used including, forexample, metals such as aluminum, copper, brass, zinc, nickel, stainlesssteel, chromium, molybdenum, vanadium, indium, titanium, gold, andplatinum; alloys formed of two or more of the metals; a conductive filmobtained by forming a conductive layer containing one or more ofaluminum, aluminum alloy, tin oxide, gold, indium oxide, etc. on afilm-like substrate such as of synthetic resin film, metal film, andpaper; and a resin composition containing conductive particles and/orconductive polymers. As the film-like substrate used for the conductivefilm, a synthetic resin film is preferred and a polyester film isparticularly preferred. Further, as the method of forming the conductivelayer in the conductive film, vapor deposition, coating, etc. arepreferred.

The photosensitive layer is formed, for example, by stacking a chargegenerating layer containing a charge generating substance, and a chargetransporting layer containing a charge transporting substance. In thiscase, an undercoat layer is preferably formed between the conductivesubstrate and the charge generating layer or the charge transportinglayer. Provision of the undercoat layer can provide an advantage ofcovering the injury and irregularities present on the surface of theconductive substrate to smooth the surface of the photosensitive layer,preventing degradation of the chargeability of the photosensitive layerduring repetitive use, and improving the charging property of thephotosensitive layer under a low temperature and/or low humiditycircumstance.

The charge generating layer contains as a main ingredient a chargegenerating substance that generates charges under irradiation of light,and optionally contains known binder resin, plasticizer, sensitizer,etc. As the charge generating substance, those used customarily in therelevant field can be used including, for example, perylene pigmentssuch as perylene imide and perylenic acid anhydride; polynuclear quinonepigments such as quinacridone and anthraquinone; phthalocyanine pigmentssuch as metal and non-metal phthalocyanines, and halogenated non-metalphthalocyanines; squalium dyes; azulenium dyes; thiapylirium dyes; andazo pigments having carbazole skeleton, strylstylbene skeleton,triphenylamine skeleton, dibenzothiophene skeleton, oxadiazole skeleton,fluorenone skeleton, bisstylbene skeleton, distyryloxadiazole skeleton,or distyryl carbazole skeleton. Among them, non-metal phthalocyaninepigments, oxotitanyl phthalocyanine pigments, bisazo pigments containingfluorene rings and/or fluorenone rings, bisazo pigments containingaromatic amines, and tris azo pigments have high charge generationability and are suitable for obtaining a light sensitive layer at highsensitivity. The charge generating substance can be used alone or two ormore of the materials can be used in combination. The content of thecharge generating substance is, without a particular restriction,preferably from 5 to 500 parts by weight and more preferably from 10 to200 parts by weight based on 100 parts by weight of binder resin in thecharge generating layer.

Also as the binder resin for the charge generating layer, those usedcustomarily in the relevant field can be used including, for example,melamine resin, epoxy resin, silicone resin, polyurethane, acryl resin,vinyl chloride-vinyl acetate copolymer resin, polycarbonate, phenoxyresin, polyvinyl butyral, polyallylate, polyamide, and polyester. Thebinder resin can be used alone or, optionally, two or more of the resinscan be used in combination.

The charge generating layer can be formed by dissolving or dispersing acharge generating substance, binder resin and, optionally, aplasticizer, a sensitizer, etc. each in an appropriate amount in anappropriate organic solvent which is capable of dissolving or dispersingthe ingredients described above, to thereby prepare a coating solutionfor charge generating layer, and then applying the coating solution forcharge generating layer to the surface of a conductive substrate,followed by drying. The thickness of the charge generating layerobtained in this way is, without a particular restriction, preferablyfrom 0.05 to 5 μm and more preferably from 0.1 to 2.5 μm.

The charge transporting layer stacked over the charge generating layercontains as essential ingredients a charge transporting substance havingan ability of receiving and transporting charges generated from thecharge generating substance, and binder resin for the chargetransporting layer, and optionally contains known antioxidant,plasticizer, sensitizer, lubricant, etc. As the charge transportingsubstance, those used customarily in the relevant field can be usedincluding, for example, electron donating materials such as poly-N-vinylcarbazole and derivatives thereof, poly-γ-carbazolyl ethyl glutamate andderivatives thereof, pyrene-formaldehyde condensation product andderivatives thereof, polyvinylpyrene, polyvinyl phenanthrene, oxazolederivatives, oxadiazole derivatives, imidazole derivatives,9-(p-diethylaminostyryl)anthracene,1,1-bis(4-dibenzylaminophenyl)propane, styrylanthracene,styrylpyrazoline, pyrazoline derivatives, phenyl hydrazones, hydrazonederivatives, triphenylamine compounds, tetraphenyldiamine compounds,triphenylmethane compounds, stylbene compounds, and azine compoundhaving 3-methyl-2-benzothiazoline ring; and electron accepting materialssuch as fluorenone derivatives, dibenzothiophene derivatives,indenothiopnene derivatives, phenanthrenequinone derivatives,indenopyridine derivatives, thioquisantone derivatives,benzo[c]cinnoline derivatives, phenazine oxide derivatives,tetracyanoethylene, tetracyanoquinodimethane, promanyl, chloranyl, andbenzoquinone. The charge transporting substance can be used alone or twoor more of the materials can be used in combination. The content of thecharge transporting substance is, without a particular restriction,preferably from 10 to 300 parts by weight and more preferably from 30 to150 parts by weight based on 100 parts by weight of the binder resin inthe charge transporting substance.

As the binder resin for charge transporting layer, it is possible to usematerials which are used customarily in the relevant field and capableof uniformly dispersing the charge transporting substance, including,for example, polycarbonate, polyallylate, polyvinylbutyral, polyamide,polyester, polyketone, epoxy resin, polyurethane, polyvinyl ketone,polystyrene, polyacrylamide, phenolic resin, phenoxy resin, polysulfoneresin, and copolymer resins thereof. Among them, in view of the filmforming property, and the wear resistance, electrical characteristicsetc. of the obtained charge transporting layer, it is preferable to use,for example, polycarbonate which contains bisphenol Z as the monomeringredient (hereinafter referred to as “bisphenol Z polycarbonate”), anda mixture of bisphenol Z polycarbonate and other polycarbonate. Thebinder resin can be used alone or two or more of the resins can be usedin combination.

The charge transporting layer preferably contains an antioxidanttogether with the charge transfer material and the binder resin forcharge transporting layer. Also for the antioxidant, those usedcustomarily in the relevant field can be used including, for example,Vitamin E, hydroquinone, hindered amine, hindered phenol, paraphenylenediamine, arylalkane and derivatives thereof, organic sulfur compounds,organic phosphorus compounds, etc. The antioxidant can be used alone ortwo or more of the antioxidants can be used in combination. The contentof the antioxidant is, without a particular restriction, from 0.01 to10% by weight and preferably from 0.05 to 5% by weight based on thetotal amount of the ingredients constituting the charge transportinglayer.

The charge transporting layer can be formed by dissolving or dispersinga charge transporting substance, binder resin and, optionally, anantioxidant, a plasticizer, a sensitizer, etc. each in an appropriateamount in an appropriate organic solvent which is capable of dissolvingor dispersing the ingredients described above, to thereby prepare acoating solution for charge transporting layer, and applying the coatingsolution for charge transporting layer to the surface of a chargegenerating layer followed by drying. The thickness of the chargetransporting layer obtained in this way is, without a particularrestriction, preferably from 10 to 50 μm and more preferably from 15 to40 μm.

Note that it is also possible to form a photosensitive layer in which acharge generating substance and a charge transporting substance arepresent in one layer. In this case, the kind and content of the chargegenerating substance and the charge transporting substance, the kind ofthe binder resin, and other additives may be the same as those in thecase of forming separately the charge generating layer and the chargetransporting layer.

In the embodiment, as described above, there is used a photoreceptordrum in which an organic photosensitive layer using the chargegenerating substance and the charge transporting substance is formed. Itis, however, also possible to use, instead of the above photoreceptordrum, a photoreceptor drum in which an inorganic photosensitive layerusing silicon or the like is formed.

The charging apparatus 1 being used is the charging apparatus 1 shown inFIG. 1 and FIG. 2. The charging apparatus 1 is disposed along alongitudinal direction of the photoreceptor drum 19 so that the chargingapparatus 1 faces the photoreceptor drum 19. The exposure unit 63exposes, according to image information of respective colors, thesurface of the photoreceptor drum 19 which has been charged by thecharging apparatus 1 to a uniform potential, to thereby form anelectrostatic latent image on the surface of the photoreceptor drum 19.

The developing apparatus 20 comprises a developer tank 17 and a tonerhopper 18. The developer tank 17 is disposed so as to face the surfaceof the photoreceptor drum 19, and supplies toner to the electrostaticlatent image formed on the surface of the photoreceptor drum 19, todevelop the image so that a visible image, i.e., a toner image isformed. Inside the developer tank 17, a developing roller is rotatablydisposed at an opening portion of the developer tank, more specificallyat a position opposed to the photoreceptor drum 19. The developingroller is a roller-shaped member for supplying the toner to theelectrostatic latent image on the photoreceptor drum 19. Moreover,together with the developing roller, a supply roller and a stirringroller are provided. The supply roller is a roller-shaped member whichis opposed to the developing roller and disposed rotatably, and used tosupply the toner to the periphery of the developing roller. The stirringroller is a roller-shaped member which is opposed to the supply rollerand disposed rotatably, and used to supply to the periphery of thesupply roller toner being newly supplied from the toner hopper 18 intothe developer tank 17. The toner hopper 18 is provided so that a tonerrefill port (not shown) disposed at a lower portion in a verticaldirection of the toner hopper 18 and a toner receiving port (not shown)disposed at an upper portion in a vertical direction of the developertank 17 are communicated with each other. The toner hopper 18 refillsthe developer tank 17 with toner according to a condition of tonerconsumption in the developer tank 17.

As the toner being used in this case, any toner used customarily in therelevant field can be used. For example, toner containing binder resin,a colorant, a charge control agent, a release agent, etc. can be used.

As the binder resin, those used customarily in the relevant field can beused including, for example, a styrene-based copolymer, polyvinylchloride, phenolic resin, a naturally modified phenolic resin, naturallymodified maleic resin, acrylic resin, methacrylic resin, polyvinylacetate, silicone resin, polyester, polyurethane, polyamide resin, furanresin, epoxy resin, xylene resin, polyvinyl butyral, terpene resin,coumarone-indene resin, and petroleum resin.

As the colorant, those used customarily in the relevant field can beused including, for example, a colorant for yellow toner, a colorant formagenta toner, a colorant for cyan toner, and a colorant for blacktoner. Examples of the colorant for yellow toner include azo pigmentssuch as CI pigment yellow 1, CI pigment yellow 5, CI pigment yellow 12,CI pigment yellow 15, and CI pigment yellow 17; inorganic pigments suchas yellow iron oxide and yellow ochre; nitro dyes such as CI acid yellow1; and solvent dyes such as CI solvent yellow 2, CI solvent yellow 6, CIsolvent yellow 14, CI solvent yellow 15, CI solvent yellow 19, and CIsolvent yellow 21, which are classified according to the color index.Examples of the colorant for magenta toner include CI pigment red 49, CIpigment red 57, CI pigment red 81, CI pigment red 122, CI solvent red19, CI solvent red 49, CI solvent red 52, CI basic red 10, and CIdisperse red 15, which are classified according to the color index.Examples of the colorant for cyan toner include CI pigment blue 15, CIpigment blue 16, CI solvent blue 55, CI solvent blue 70, CI direct blue25, and CI direct blue 86, which are classified according to the colorindex. Examples of the colorant for black toner include carbon blackssuch as channel black, roller black, disc black, gas furnace black, oilfurnace black, thermal black, and acetylene black. Among the variouscarbon blacks, a suitable carbon black may be appropriately selectedaccording to an intended design characteristic of toner.

The colorant can be used alone or two or more of the materials can beused in combination. Further, two or more materials of the same colorseries can be used, and one or two or more materials of different colorseries can be used.

A usage of the colorant is, without a particular restriction, preferably5 to 20 parts by weight based on 100 parts by weight of the binderresin. By using the colorant of which amount is in the above range,various properties of toner are not deteriorated, and it is thuspossible to form an image of high density and very high quality.

As the charge control agent, it is possible to use those for positivecharge control and for negative charge control, which are usedcustomarily in the relevant field. Examples of the charge control agentfor positive charge control include basic dye, quaternary ammonium,aminopyrine, a pyrimidine compound, a polynuclear polyamino compound,aminosilane, and a nigrosine dye. Examples of the charge control agentfor negative charge control include solvent dyes such as oil black andspilon black; a metal-contained azo compound; metal salt naphthenate;metal salt salicylate; a fatty acid soap; and a resin acid soap. Thecharge control agent can be used alone or two or more of the materialscan be used in combination. A usage of the charge control agent can be,without a particular restriction, appropriately selected from a widerange and is preferably 0.5 to 3 parts by weight based on 100 parts byweight of the binder resin.

As the release agent, those used customarily in the relevant field canbe used including, for example, petroleum wax such as paraffin wax, aderivative thereof, microcrystalline wax, and a derivative thereof;hydrocarbon synthetic wax such as Fischer-Tropsch wax, a derivativethereof, polyolefin wax, a derivative thereof, low-molecular-weightpolypropylene wax, a derivative thereof, low-molecular-weightpolyethylene wax, and a derivative thereof; vegetable-based wax such ascarnauba wax, a derivative thereof, rice wax, a derivative thereof,candelilla wax, a derivative thereof, and haze wax; animal-based waxsuch as bees wax and whale wax; oil and fat-based synthetic wax such asfatty acid amide and phenolic fatty acid ester; long-chain carboxylicacid, and derivative thereof; and long-chain alcohol and a derivativethereof. Note that the derivatives include an oxide, a block copolymerof vinyl-based monomer and wax, and a graft modification. A usage of thewax can be, without a particular restriction, appropriately selectedfrom a wide range and is preferably 0.2 to 20 parts by weight based on100 parts by weight of the binder resin.

Furthermore, a fluidity improver may be contained as an externaladditive. The fluidity improver exerts an effect thereof when attachedto, for example, a surface of toner. As the fluidity improver, thoseused customarily in the relevant filed can be used including, forexample, silica, titanium oxide, silicon carbide, and aluminum oxide.The fluidity improver may have a surface thereof treated with ahydrophobic process by use of, for example, polyorganosiloxane having atrimethylsilyl group. The hydrophobic process is preferably applied tosilica or the like material. The hydrophobized fluidity improver,particularly hydrophobized silica, is generally attached to an electrodeetc. of a charging apparatus, and often decreases a charging ability ofthe charging apparatus for a photoreceptor drum, resulting in a chargingdefect. By contrast, when the charging apparatus 1 of the invention isused, even an image formation using toner containing hydrophobizedsilica lead no charging defect and there are thus caused no defects inimages. The fluidity improver can be used alone or two or more of thematerials can be used in combination. A usage of the fluidity improveis, without a particular restriction, preferably 0.1 to 3.0 parts byweight based on 100 parts by weight of the toner particles.

After the toner image is transferred onto the recording medium by thetransfer unit 65, the cleaning unit 64 cleans the surface of thephotoreceptor drum 19 by removing the toner remaining on the surface ofthe photoreceptor drum 19. For the cleaning unit 64, a plate-shapedmember such as a cleaning blade is used. In the image forming apparatusaccording to the invention, an organic photoreceptor drum is usedpredominantly for the photoreceptor drum 19, and since the surface ofthe organic photoreceptor drum mainly contains a resin ingredient, thesurface tends to be degraded by the chemical action of ozone generatedby the corona discharge by the charging apparatus. However, the degradedsurface portion is worn under the frictional rubbing effect brought bythe cleaning unit 64, and is removed reliably although gradually.Accordingly, the problem of the degradation of the surface by the ozoneor the like can be actually overcome, and the potential of the surfacebeing charged through the charging operation can be maintained stablyfor a long time.

The transfer unit 65 is located above the photoreceptor drum 19, andcomprises a transfer belt 72, a transfer belt driving roller 73, atransfer belt driven roller 74, a transfer roller 71 (b, c, m, and y),and a transfer belt cleaning unit 75. The transfer belt 72 is tightlystretched over the transfer belt driving roller 73, the transfer beltdriven roller 74, and the transfer roller 71. Rotation of the transferbelt driving roller 73 being driven works on the transfer belt 72 so asto be driven to rotate in an arrow B direction.

The transfer belt 72 driven to rotate in the arrow B direction is anintermediate transfer belt, and disposed in contact with the respectivephotoreceptor drums 19. When the transfer belt 72 passes by thephotoreceptor drum 19 in a state of being in contact therewith, there isapplied a transfer bias of which polarity is opposite to the polarity ofthe charged toner on the surface of the photoreceptor drum 19, from thetransfer roller 71 which is disposed opposite to the photoreceptor drum19 across the transfer belt 72, with the result that the toner imageformed on the surface of the photoreceptor drum 19 is transferred ontothe transfer belt 72. In the case of full-color image, toner images ofrespective colors formed at the respective photoreceptor drums 19 aresequentially stacked on the transfer belt 72 so that a full-color imageis formed.

The transfer belt cleaning unit 75 is disposed so as to be opposite tothe transfer belt driven roller 74 and in contact with an outercircumferential surface of the transfer belt 72 tightly stretched overthe transfer belt driven roller 74. Since the toner attached to thetransfer belt 72 through contact with the photoreceptor drum 19 maycause a contamination on a backside of the recording medium, thetransfer belt cleaning unit 75 removes and collects the toner on thesurface of the transfer belt 72.

The recording mediums, e.g. recording paper, on which the toner imagesare to be recorded, are stored in the automatic paper feed tray 67. Inthe image forming apparatus 61 of the invention, the automatic paperfeed tray 67 is disposed in a lower portion of the apparatus. Therecording paper stored in the automatic paper feed tray 67 is taken outof the automatic paper feed tray 67 sheet by sheet to be thereafter fedto the sheet conveying channel S by the pickup rollers 78. The recordingpaper fed to the sheet conveying channel S is conveyed by a plurality ofconveying rollers 81 disposed at various positions of the sheetconveying channel S, and in synchronization with a position of theformed image which has been transferred on the transfer belt 72 at thetransfer unit 65, the recording paper is fed to a nip portion betweenthe transfer belt driving roller 73 and a recording paper transferroller 79 which is disposed so as to face the transfer belt drivingroller 73 and be pressed on the transfer belt driving roller 73. As aresult of application of the transfer bias from the recording papertransfer roller 79 to the recording paper passing through theabove-described nip portion, the toner image is transferred from thetransfer belt 72 onto the recording paper. Note that the feeding of therecording paper is not restricted to the feeding from the automaticpaper feed tray 67 but may be from the manual paper feed tray 68 throughanother sheet conveying channel S.

The fixing unit 66 is disposed downstream of the transfer unit 65 in aconveying direction of recording paper, and comprises a heating roller76 and a pressure roller 77, a heat source of the heating roller 76, asensor detecting a temperature of the heating roller 76, and a controlportion for controlling the heat source to operate so that the heatingroller 76 attains a predetermined temperature. The heating roller 76 andthe pressure roller 77 are disposed so that the recording paper nippedunder pressure between the heating roller 76 and pressure roller 77 in amutually pressed state can be conveyed. In the fixing unit 66, when therecording paper passes through the nip portion formed between theheating roller 76 and the pressure roller 77, the toner image is heatedand pressurized to be fixed on the recording paper so that a solidrecording image is formed.

The recording paper on which the toner image has been fixed in thefixing unit 66, is discharged to the paper discharge station 69 by apaper discharge roller 80 and a conveying roller 81 which are disposedat an exit side of the fixing unit 66.

Hereinbelow, an image forming operation in the image forming apparatus61 will be briefly described. In an image forming section, the surfaceof the photoreceptor drum 19 is charged to a uniform potential by thecharging apparatus 1, and according to the image information, theexposure unit 63 exposes the image to form the electrostatic latentimage which is then developed by the developing apparatus 20 to form thetoner image. The toner images of respective colors formed on thesurfaces of the respective photoreceptor drums 19 are sequentiallystacked on the transfer belt 72 so that the full-color image is formed.

The toner image transferred on the toner belt 72 is transferred onto therecording paper which has been picked up from the automatic paper feedtray 67 by the pickup roller 78 and conveyed in the sheet conveyingchannel S to the nip portion between the transfer belt driving roller 73and the recording paper transfer roller 79. The recording paper on whichthe toner image has been transferred, is conveyed to the fixing unit 66where the toner image is then treated with a fixing process into thesolid recording image, and discharged to the paper discharge station 69where a series of image forming operation comes to the end.

The image forming apparatus 61 is provided with the charging apparatus1. This makes it possible to prevent the charging defects of thephotoreceptor drum 19, with the result that images of high quality andhigh grade can be formed over a long period of time.

EXAMPLE

The invention will be described specifically with reference to examples.

Example 1

To a plate metal formed of stainless steel (SUS304) (sized 20 mm×310mm×0.1 mm thickness) were applied masking and etching to prepare aneedle electrode substrate. The etching was conducted by spraying anaqueous 30 wt % solution of ferric chloride to the plate metal ofstainless steel at a liquid temperature of 90° C. for 2 hours. After theetching, the needle electrode substrate was taken out of the etchingsolution, and water washing and cleaning with pure water were conductedto manufacture a needle electrode substrate.

An Ni plating layer of 0.5 μm thickness was formed by electric platingon the surface of the needle electrode substrate obtained as describedabove. And then, the needle electrode substrate having the Ni platinglayer formed thereon was dipped into a boron-containing nickel solution(trade name of products: Composite, Sumer SC-93 manufactured by JapanKanigen Co., Ltd., liquid temperature: 90° C.) for 60 min to manufacturea needle electrode, on a surface of which a boron-containing nickelplated layer of 10 μm thickness was formed. After the plating, theneedle electrode-was taken out of the plating bath, and water washingand cleaning with pure water were conducted, followed by drying. Asurface of the plated layer was observed by electron microscopy, and itwas confirmed that the surface was very smooth.

The needle electrode (sawtooth electrode) replaced a needle electrode ofa charging apparatus in a commercially available image forming apparatus(trade name of products: AR625, manufactured by Sharp Corp.) tomanufacture an image forming apparatus including the charging apparatusof the invention. The following test was conducted by use of the needleelectrode, the charging apparatus and the image forming apparatus.

[Discharging Test]

As a test under harsh conditions, an aging test with no paper passageunder a low humidity condition (10% or lower) was conducted. Since AR625is a 70 sheet machine, 71 hours corresponds to the number of copies(300K life). In the test, the potential of the charged surface of thephotoreceptor drum was set to −630 V at the initial state.

[Detection of Nitrogen Oxide and Rust]

The rust and nitrogen oxides were detected by microscopic observation onthe needle electrode after discharging.

As a result, no deposition of rust etc. was found on the plated needleelectrode according to the invention whereas a deposition of rust etc.is found on a needle electrode left in the state of the stainlessmaterial as it was.

Also after printing 300000 sheets in an actual copying test, whitestreaks or black streaks were observed in half-tone images in the casewhere the cleaning is not performed on the needle electrode left in thestate of the stainless material as it was whereas the quality ofhalf-tone images was uniform and no unevenness occurred in the platedneedle electrode of the invention.

[Extraneous Substance Attachment Test]Inside an about 1 m³volume-chamber for acceleration test, hydrophobic silica was heated to120° C. or more to contaminate an interior portion of the chamber. Thecharging apparatus of the invention was placed in the chamber andcontinuously discharged electricity for 2 hours. An extraneous substancewas forcibly attached to tips of the needle electrode (sawtoothelectrode) to cause a charging defect. After that, the chargingapparatus of the invention was taken out of the chamber, and a rubbingcleaning was conducted by a cleaner provided in the charging apparatus.By so doing, the extraneous substance was removed from the tips of theneedle electrode, resulting in recovery of favorable charging propertyof the charging apparatus. The charging apparatus was incorporated intoa commercially available image forming apparatus (trade name of product:AR625, manufactured by Sharp Corp.), and an image was formed therewith.It was then confirmed that the apparatus was able to offer an image offavorable quality, which was evenly charged without variation.

A needle electrode (sawtooth electrode) on which a gold plated layer hadbeen formed instead of the boron-containing nickel plated layer, wasmounted in the charging apparatus, and in the same manner as describedabove, the extraneous substance (hydrophobic silica) was forciblyattached to the needle electrode. In this case, even when the cleaningwas conducted, the charging property was not recovered and just anunevenly charged image was formed.

Further, it was found that, owing to the smoothness on the surfacerecognized as the feature of the boron-containing nickel plating,deposits such as dusts in air were decreased compared to the needleelectrode left in the state of the stainless material as it was, and thedusts were able to be removed easily by cleaning. On the contrary, inthe case where only the Ni plated layer is applied, contaminants couldnot be removed sufficiently by cleaning and therefore, the quality ofhalf-tone images was not recovered uniformly after cleaning.

Note that, regarding polytetrafluoroethylene powder-containing nickelelectroless plating of which cost is lower than the boron-containingnickel plating, an allowable temperature limit is lower than that of theboron-containing nickel plating, but an appropriate selection of aplating condition, e.g. application of plating having a thickness of 10μm, enables to make the surface of the plated layer smooth. Thepolytetrafluoroethylene powder-containing nickel electroless plating isthus effective for securing the cleaning performance.

The invention may be embodied in other specific forms without departingfrom the spirit or essential characteristics thereof. The presentembodiments are therefore to be considered in all respects asillustrative and not restrictive, the scope of the invention beingindicated by the appended claims rather than by the foregoingdescription and all changes which come within the meaning and the rangeof equivalency of the claims are therefore intended to be embracedtherein.

1. A charging apparatus comprising: an electrode having a plurality ofpointed protrusions, that applies a voltage to a surface of aphotoreceptor drum to charge the surface; and a platy grid disposedbetween the electrode and the photoreceptor drum, that controls apotential of the surface of the photoreceptor drum being charged,wherein a nickel layer containing boron is formed on at least one ofsurfaces of the electrode.
 2. The charging apparatus of claim 1, whereinthe nickel layer containing boron is formed by an electroless platingmethod.
 3. The charging apparatus of claim 1, wherein a thickness of thenickel layer containing boron is 0.3 μm or more.
 4. The chargingapparatus of claim 1, wherein another nickel layer is formed between theelectrode and the nickel layer containing boron.
 5. The chargingapparatus of claim 1, wherein the nickel layer containing boron containsphosphorus together with boron.
 6. An image forming apparatuscomprising: a photoreceptor drum on a surface of which an electrostaticlatent image is formed; the charging apparatus of claim 1, for chargingthe surface of the photoreceptor drum; an exposure unit that irradiatesthe charged surface of the photoreceptor drum with signal light based onimage information to thereby form the electrostatic latent image; adeveloping apparatus that develops the electrostatic latent image formedon the surface of the photoreceptor drum to thereby form a toner image;a transfer unit that transfers the toner image onto a recordingmaterial; and a fixing unit that fixes the toner image transferred onthe recording material.
 7. The image forming apparatus of claim 6,further comprising a cleaning unit that cleans the surface of thephotoreceptor drum after the toner image has been transferred onto therecording material by the transfer unit, wherein the developingapparatus and/or the cleaning unit are/is located above the chargingapparatus.
 8. The image forming apparatus of claim 6, wherein the tonerimage is formed of toner which contains hydrophobic silica as anexternal additive.